The present invention relates to a process for producing a sensor membrane substrate, in particular, for a mass flow sensor or a pressure sensor, the substrate having a membrane at its front, which is fixed at the edge of an opening that is etched free from the back.
Although applicable to the manufacture of any kind of sensor membrane substrates, the present invention as well as the underlying difficulty are explained with respect to a mass flow sensor, e.g., an air-mass flow sensor for use in automotive engineering, using silicon technology.
In the case of such a customary air-mass flow sensor, the air mass is measured thermoelectrically on such a thin dielectric membrane. The membrane is manufactured by depositing functional membrane layers on the front of the substrate (silicon wafer) and subsequently etching free the membrane area from the back.
FIG. 4 is a schematic representation of a customary sensor membrane substrate, illustrating the difficulty arising in the case of the usual sensor membrane substrate.
In FIG. 4, reference symbol 10 refers to a silicon substrate having a front VS and a back RS, 20 and 25 to membrane layers of SiO2 and Si3N4, respectively, 50 to an opening etched from the back, R to an edge of opening 50 and 100 to the membrane, and A to a fixing area of membrane 100.
A disadvantage of the above known approach is that, due to the etching process, a crystallographically-inherent, sharp etching edge forms at the transition of the membrane/silicon substrate, i.e., at edge R. When pressure is exerted from front VS, a notch effect can arise at this etching edge which is so great that stress cracks can form in the individual membrane layers.
German Patent No. DE 42 15 722 suggests introducing additional doping areas on the front in the area of the edge. These are not etched during the anisotropic etching of the back to clear membrane 100, and are rounded during a re-etching process with an etching solution which etches silicon isotropically, in order to reduce the notch stress and increase the compressive strength. However, this procedure is costly from the standpoint of process technology, since it requires an additional doping step and etching step.
The process according to the present invention for producing a sensor membrane substrate has the advantage over the known approach, in that only one additional thickening step is necessary without requiring an additional etching step.
A basic idea of the present invention lies in locally thickening the substrate in an area on the front opposite the edge, the thickened portion having a continuous transition to the substrate. Subsequently, a membrane layer is deposited on the front having the locally thickened area. The rear etching edge is then placed under the thickened area.
Such a gentle, smooth transition can reduce the notch effects and the tendency to fracture in the fixing area when pressure is applied, and the stability of the membrane is increased, since the tensions are distributed more favorably in the fixing area of the membrane. No stress cracks appear any longer, and the compressive strength is decisively improved.
According to a preferred embodiment, the step of local thickening includes the following steps: depositing and patterning of a masking layer on the front of the substrate to lay bare the area opposite the edge and local oxidizing of the area. The beak-shaped transitions during local oxidizing are excellently suited for making an even transition between the thickened and the unthickened area. Furthermore, the local oxidation of silicon is a process which can be well controlled.
According to a further preferred embodiment, prior to the local oxidation, a lower membrane sublayer is made by oxidizing the substrate, and subsequently, the thickening is performed in the lower membrane sublayer. However, it is absolutely possible to omit this membrane sublayer.
According to a further preferred embodiment, the masking layer is removed prior to depositing a membrane layer on the front. This is done if the intention is not to embed the masking layer in the membrane. However, such an incorporation or the incorporation after a conversion (e.g., returning a nitride layer into an oxide layer) is possible.
According to a further preferred embodiment, the substrate is a silicon substrate.
According to another preferred embodiment, the masking layer is a nitride layer.
According to a further preferred embodiment, the masking layer is also used for masking the back during the etching of the opening. Thus, this layer can have a dual function.
According to yet another preferred embodiment, the locally oxidized area, except for the membrane area, is located on the remaining substrate.